District heating is an important pillar of the energy supply in Germany. The energy suppliers operate and maintain a pipeline network of more than 26,000 kilometers. For this part of the predominantly underground infrastructure, there is still no established trenchless rehabilitation process. In the case of renovation, the pipes must be replaced in an open construction.
“There is a need for rehabilitation and the network operators are very interested in suitable methods,” says Sebastian Grimm, who is responsible for research and development at the AGFW industry association. For almost five years, Grimm has been dealing with the topic of possibilities and limits of on-site hardening hose liner rehabiltations in district heating pipes. He accompanied a research project funded by the Federal Ministry of Economics and Climate Protection, which ended in February 2022.
Stadtwerke Neumünster were participants in the joint project. Their task was to set up a test track. For this purpose, an above-ground bypass of an underground main line of the Neumünster district heating network, which is always in operation, was built on a brownfield site in the city of Neumünster. The test line is a steel line with a total length of 115 meters and a diameter of DN 200.
The research project with a glass fiber liner, which was completed in February, has not yet produced any practical results. “We are currently applying for another research project in order to stay on the ball with this topic,” explains Sebastian Grimm.
Independent of the research project, 10 years ago the company Pressure Pipe Relinig Sweden AB, PPR, began to develop a hose liner for use in district heating pipes. In 2014, the Swedish high-tech specialist for the processing of carbon fibers, Oxeon, and in 2017 the liner manufacturer BKP Berolina from Velten near Berlin joined as development partners. Under the product name CarboSeal, the new liner 2021 was used for the first time in two projects in Sweden and is now also being offered in Germany as a marketable product.
Against this background, the idea arose to use a 40-meter-long segment of the existing and currently unused test line in Neumünster for the installation of the CarboSeal liner, in order to assess its practical suitability from installation to the rehabilitation result and grid operation, with the support of the industry association AGFW investigate. For this purpose, the line was provided with imperfections such as defective welding seams and simulated corrosion damage.
The fact that the completed research project did not succeed in arriving at practicable results points to the difficulties that need to be solved when renovating district heating pipes. “The main challenges are the high temperatures and the temperature differences that we have to deal with when the lines are in operation,” explains Lars Quernheim, who is responsible for research and development and technology at BKP.
When PPR in Sweden started developing a liner for district heating pipes about 10 years ago, the first step was to look for the temperature conditions to be encountered. “It probably took two to three years before we found what we were looking for,” says Quernheim. A high-temperature resistant, two-component, heat-curing epoxy resin is used. In the next step, tests were made with different reinforcement materials and finally the carbon fabric Textreme from Oxeon was chosen as the optimal carrier material. “The carbon fiber offers the possibility of adapting the temperature-related linear expansion of the liner of the steel pipes to be renovated. As a result, we have no or only an extremely small relative movement between the liner and the steel pipe,” Lars Quernheim continues. After finding the relevant materials, the next challenge is to fabricate a working liner from the components. BKP in Velten has been dealing with this task for the last three years. By mid-2021, together with PPR Sweden, a result was achieved that today makes it possible to mass-produce the product. The result is the CarboSeal liner.
In Velten, the carbon fiber fabric manufactured by Oxeon is coated with epoxy resin and folded into a tube. The structure with inner and outer film is largely based on that of the well-known Berolin-Liner. After a short maturation phase, the product is frozen. The liner can be stored for several months at temperatures between -15 and -25 degrees.
For the installation, the liner is thawed and, as is typical for the hose liner process, drawn in and set up. Curing takes place with steam. There are restrictions with regard to the liner’s ability to bend, and connections must be made in an open design using head holes.
In any case, CarboSeal was able to make a positive impression on the representatives of municipal utilities from the region who had traveled to Neumünster. The installation by a team from Sweden and Velten took place without any problems. “It’s a simple, quick, clean process,” says Jörg Stoltenberg, head of district heating at Stadtwerke Neumünster. “Of course we have optimal installation conditions here on the test site, but I see great potential for leaking pipes and especially where we cross railway lines or busy roads, the liner is an interesting alternative to replacing the pipes in an open construction method.”
Sebastian Grimm also took home interesting impressions from Neumünster for the AGFW and its member companies. The installation was documented in a video produced on site by the AGFW, which is intended to serve as independent information material for interested parties. “We already have inquiries from member companies who want to test the process in their own network, and then we will certainly be able to gather more experience in the near future.”
All these signs make Marco Warkotz-Gruber, who is in charge of CarboSeal sales for PPR Deutschland, optimistic. “The feedback from the market in Germany and neighboring countries is very positive, but there is also a certain reluctance to want to be the first with this product.” Sales Manager. Therefore, this test installation, accompanied by the AGFW, is of particular importance for the market launch, according to the sales manager.